This invention relates to a device for detecting broken filaments of lamps which are used for illuminating a wide area such as an airport.
When illuminating a wide area such as an airport, it is an ordinary practice to divide a plurality of such lamps as tungsten lamps, mercury lamps, halogen lamps, and the like into several groups in accordance with their application and the convenience of controls, each group being connected in series for the purpose of equalizing the light intensities and decreasing the installation cost of the lamps. More specifically, lighting transformers 3, 4, 5, and 6 are provided for respective lamps, and the primary windings of the lighting transformers 3, 4, 5, and 6 are connected in series, while the secondary windings thereof are connected respectively with lamps 7, 8, 9, and 10 as shown in FIG. 1. The serially connected transformers are connected with an AC power source 1 through a control device 2.
The control device 2 includes a current-controlling means utilizing thyristors (SCR) or a group of elements connected to a resonance circuit of, for instance, CR type including capacitors and resistors.
A voltage detecting device 11 and a current detecting device 12 are further provided in a well known manner so that the output voltage and the output current delivered from the control device 2 can be detected.
The outputs of the devices 11 and 12 are supplied to a broken filament detecting device 13 wherein the voltage and the current detected by the detecting devices 11 and 12 are rectified, and the areas defined by the waveforms of the thus rectified voltage and current are determined. The ratios of the determined areas to predetermined standard values are compared with each other, and the difference is further compared with a predetermined value. When the difference between the area ratios is greater than the predetermined value, it is judged that one or more filaments of the lamps are broken. Herein the term "filament" is defined to indicate not only a filament but also an element of a lamp, a failure thereof renders the lamp inoperative.
When the comparator detects the fact that the difference between the two areas ratios is greater than the predetermined value, the device 13 delivers an output to an alarming device 14, such as a buzzer.
FIGS. 2(a) and 2(b) are waveform diagrams for the two cases, one having no broken filament and the other having one or more filaments broken. In these figures, numeral 201 designates the waveform of a voltage detected by the voltage detecting device 11, numeral 202 the waveform of a current detected by the current detecting device 12, and numeral 203 the difference between the area ratios of the waveforms 201 and 202, which is compared with a predetermined value.
As is apparent in FIG. 2(b) showing a case in which one or more filaments are broken, the voltage waveform 201 rises up sharply while the current waveform 202 rises up slowly. For this reason, the difference 203 between the area ratios of the voltage and the current in FIG. 2(b) becomes greater than the difference 203 in FIG. 2(a), and the percentage of the lamps having broken filaments against the entire lamps (hereinafter termed "percentage of the broken filaments") can be detected by comparing the difference 203 in FIG. 2(b) with a predetermined value which has been selected to be greater than the difference 203 in FIG. 2(a).
The above described conventional broken filament detecting device 13, however, tends to deliver an erroneous output to the alarming circuit 14 when the voltage waveform 201 or the current waveform 202 is greatly deformed by external noises and the like, to which the analogue values of the voltage and the current are susceptible. For obviating this disadvantage, if a large value is selected for the predetermined value to be compared with the aforementiond difference, the sensitivity of detecting broken filaments is reduced, and the detection of the broken filaments at a high precision is made difficult.
Furthermore, it is known that the waveforms of the voltage and current are substantially affected by the characteristics of each of the lighting transformers. Thus when a faulty transformer is replaced by a new transformer, an adjustment is required for equalizing the characteristics of the entire load before and after the replacement.
In addition, various disadvantages have been revealed with the conventional device. For instance, an additional circuit is required when an addition of any other function is desired, or it is difficult to memorize various detected values temporarily when the variation of characteristics of the detected values is required to be surveyed for a considerably long period.